49:5-6 This rhetorical question sets forth the folly of fearing when wicked people oppose the righteous. It introduces the revelation that the prosperous ungodly enjoy a false security (vv. 7-12).
49:7-9 Material wealth cannot prevent death. No one has enough money to buy life back when God claims it in death. The point here is that we cannot buy our way, or anyone else's way, out of dying. The psalmist was not speaking of purchasing eternal salvation here. This comes later in verse 15 (cf. Matt. 20:28).
49:10-12 Everyone dies eventually even though some live with the illusion of immortality. The fact that people try to perpetuate their reputations on the earth forever shows they want to live forever. However, man, like the animals, will eventually go into the grave. Of course, the psalmist did not mean that man's fate is identical to that of animals in all respects. He only meant both die. Later revelation that saints living at the Rapture will experience translation without dying does not negate the psalmist's point.